In a known cut-and-weld device (German Offenlegungsschrift No.1,454,991) two foils to be welded together are stretched over a foil support between adjacently extending pairs of driver belts over a groove in the foil support into which a cut-and-weld cutter extends to provide the two foils with a separating weld seam. The pairs of driver belts produce a certain tensioning of the foils transversely of the feed direction; however, in many cases this tensioning is insufficient to produce a clean separating weld seam. In particular during rapid advance of the foils it may happen that the portions to be separated by the cut-and-weld cutter are located so close together again behind the cutter that when the foil material which had been softened by the cut-and-weld cutter solidifies, a connection * of the adjacently guided strips occurs again, so that the desired separation does not take place.
In order to provide a remedy therefor, attempts have already been made to lower a tensioning member from above upon the foil behind the cut-and-weld cutter in the feed direction, and to cause it to engage into groove in the foil support. Thereby a certain additional tension results transversely of the feed direction and an improved separation of the foil portions; however, new problems are produced thereby. Thus such a tensioning element which is frequently constructed in the form of a tension rail easily causes an accumulation of material behind the cut-and-weld cutter, such accumulation being due to the fact that the foil portions fed towards the tension rail do not travel smoothly past the latter, but adhere thereto. For this reason such a device can be operated only at relatively low feed speeds. Moreover, even with such tensioning elements a satisfactory separation of the foil portions behind the cut-and-weld cutter is still not obtained.